The present invention concerns a process and relative production line for the direct manufacture of finished pressed or deep drawn pieces from ultrathin hot strip cast and rolled in-line.
Worldwide production of hot rolled strip for the manufacture of cold rolled strip amounts to about 40% of world steel production which is currently about 750 million tonnes/year. This share of cold rolled steel production in industrial countries amounts to about 50%, however, from which it can be deduced that the growth potential for hot or cold rolled strip production is very high at a worldwide level.
At the same time it must be remembered that the investment costs for traditional production lines are very high and, on the basis of an integral cycle steel mill with a capacity of about 4 million tonnes/year, expressed in specific investment costs, amount to about US$ 1000/tonne of cold rolled strip.
A traditional process and production line for the manufacture of cold rolled strip with gauges from 0.6 to 0.1 mm, coated or non-coated, is composed for example, as shown in FIG. 1 relative to the prior art, of:
blast furnace production (1);
oxygen melt shop (2)xe2x80x94convertor;
continuous slab casting plant with thickness 200-250 mm and width 800-2600 mm (3);
hot rolling mill (4) composed of a furnace (4.1), a roughing mill (4.2) and a finishing mill (4.3) for the manufacture of hot strip in gauges between 4 and 2 mm and a maximum width of 1800 mm for the manufacture of cold rolled strip;
continuous pickling (5);
cold rolling mill (6), for example as a continuous or reversible rolling mill for the manufacture of gauges between 0.6 and 0.3 mm;
annealing (7) of the continuous or bell type;
cold finishing mill (skinpass mill) with temperature management and control (8).
This cold rolled strip, controlled as regards thickness, crown and flatness, will feed at choice: a tinning line (9) or a galvanizing line (10) or, without surface coating, a service centre directly (11) where, depending on customer requirements, it will be transformed in the form of strip or packs of sheet, depending on the orders, to then leave the factory (12) by transport on road, rail and/or water (13). This traditional form of selling finished cold rolled strip also involves the transport of the processing scrap (16) produced by those who carry out subsequent processing (14). This scrap derives, for example, from pressing or deep drawing (15) of finished parts such as, for example, assembly components (boxes, car and tank pieces etc.). This processing scrap (16), which currently amounts to about 15% of the whole of worldwide steel production, can be seen as a xe2x80x9csteel tarexe2x80x9d which is transported uselessly from the steel manufacturer (12) to the customer (14) to return once more to the steel manufacturer (12) and consequently implies transport costs in the form of time, energy and environmental pollution.
The customer (14) traditionally collects from the steel mill sheets or coils which are suitable for deep drawing and pressing, preferably with a carbon content below 0.06%. The customer unwinds them and puts them for example into a press (15) in order to obtain products (17) such as:
press-processed pieces or
deep drawn pieces such as, for example, external or internal parts for the construction of cars or lorries.
The scrap derived from processing (16), also defined as xe2x80x9cnew scrapxe2x80x9d, produced from the pressing of sheets at the customer""s or end user""s (14) and as a share of tare already amounts to about 30% of total scrap production, which currently corresponds to about 50% of world steel production, must be transported back to steel manufacturing, such as an oxygen melt shop (2) or an electric mill (2.1), causing consequent costs. This means that the scrap makes its way again to the steel manufacturer""s to be recycled.
Moreover, this production line is characterized by a longitudinal dimension of about 1500-2000 meters and a transversal dimension of about 50 meters, calculated from the continuous casting plant (3) to shipment (11) of the cold rolled product in the form of coils (11.1) or packs of sheet (11.2). Moreover, each manufacturing phase is generally equipped with an uncoiling and coiling station which in addition causes expenditure of work, loss of energy and material, as well as possible operating anomalies, and also requires space for storage and moving the coils between one production phase and the subsequent one.
An initial shortening of the casting and rolling processes, and therefore a cost reduction in the price per tonne of hot rolled strip of about 50%, could be achieved with the introduction of the thin slab technique together with the continuous finishing mill. As regards this, the so-called ISP (In-line Strip Production) process in particular, with the components of the cast-rolling technique, i.e. slab thickness reduction during and immediately after the solidification phase, is to be cited (DE 38 40 812, DE 38 18 077, DE 44 03 048 and DE 44 03 049). This technology, compared with other thin slab technologies which show no thickness reduction, leads to a reduction of up to 50% during the solidification phase and up to 80% directly after solidification, a better surface quality and, at the same time, a finer crystalline structure, improved internal quality and, therefore, considerably improved properties of the material in the end product.
In the ISP process (23.4) for example, which is represented in part of FIG. 2, the slab casting thickness (18.3) in the thin slab casting plant (18) is reduced during solidification in the roll table (18.2) from a thickness of 65 mm on leaving the mold (18.1) to a minimum thickness of 30 mm. Directly after solidification the slab thickness is reduced to as low as 6 mm by means of a rolling process, for example through three small stands (19) with an entry speed from 0.066 to a maximum of 0.15 m/s.
These cast-rolling technologies during and directly after solidification produce slabs with very good surface characteristics and a central-symmetrical and controlled convexity (crown) for example of 1.0-1.5% on a thickness of 6-25 mm, good flatness as well as a uniform grain size structure with minimum degrees of slab deformation from 30 mm to a minimum thickness of 6 mm or with a lengthening of 5 times.
The good production of the thin slab (18.3) and above all of the intermediate strip (19.1) in its shape and structure is to be traced back to the rolling in casting during solidification and above all to the rolling process after solidification which is characterized by a considerable transversal flow of the thin slab to be rolled in the pass between the rolls. This transversal flow is caused by the low deformation speed and the low resistance to rolling in the transversal direction of the material from thin slabs. Moreover, the good behaviour of the flow of the rolling material (18.3) in the pass between the roughing mill rolls (19) is directly favoured, after solidification, by the low deformation force at the high average temperature of 1350xc2x0 C. in the cross-section of the slab. Moreover, the slab (18.3) with a surface temperature of about 1200xc2x0 C. on entry to the first rolling stand of the roughing mill (19) still has a thermal gradient, i.e. a temperature increase in the direction of the slab nucleus.
This external and internal temperature between the solidification point and entry to the first stand is controllable by cooling and favours a current of the uniform mass on the cross-section of the slab in the pass between the rolling cylinders: i.e. it allows a uniform degree of deformation on the slab thickness or better on the thickness of the material to be rolled. This intermediate product (19.1), cast and rolled during and directly after solidification, presents the following characteristics:
thickness of 6-25 mm;
width of 700-2000 mm;
central symmetrical crown between 1.0 and 1.5%;
central symmetry of the convexity  greater than 95% (wedge) on the width of the material to be rolled;
high degree of flatness of the material to be rolled;
better surface quality, which meets the high demands for the deep drawing (05/05) of external automobile parts;
uniform, homogeneous and transversally fine crystalline structure which leads to high resistance and toughness as well as excellent ductility for good cold deformation;
This intermediate rolled product (19.1) manufactured in this way with its positive characteristics, which a rolled product produced according to the prior art does not usually show, with a thickness of 25-6 mm derived from the traditional slab between points (3) and (4.6) having a thickness of 280-150 mm, or from a conventional thin slab with a thickness of 50 mm, is now heated, preferably by means of an induction furnace (21), to an optimal temperature in relation to the form of rolling which is determined by:
the steel grade;
the final rolling thickness;
management of the hot strip temperature in the rolling mill (23) between the first and the n-th stand as also in the cooling line (23.1) and in the hot rolled strip coiler (23.2);
recrystallization and formation of the structure with respect to the material and its behaviour in the T.T.T. diagram (time-temperature-transformation);
crown;
flatness
To be then taken directly or again coiled into an intermediate coil (22) at the rolling mill (23) for example by means of a form of continuous rolling without longitudinal cutting of the slab (18.3). In the rolling mill (23) the hot strip (23.3) with a thickness between 1.2 and 0.6 mm finally reaches the hot strip coiler (23.2) for recrystallization, from where it is then taken to other processing processes at the cold rolling mill (25) with or without subsequent surface coating.
The task of the invention is now that of considerably simplifying the traditional hot rolled strip production process described above and based on the traditional slab or even on a normal thin slab (FIG. 1) with the help of the ISP process (23.4) and a rolling product with a thickness of 0.6-1.2 mm, saving stages in the process, reducing costs and having the possibility of directly preparing, subsequently to the rolling process, for example finished pressed or deep drawn pieces such as details for a car door, pieces which are then supplied as finished products (32) to the end user i.e. the car manufacturer for final assembly.
This technology would lead to savings in the following sectors:
investment costs;
manufacturing costs:
energy
a material
a salaries and wages
transport
cost per piece
as well as improvements as regards environmental pollution, supported by:
savings in the annealing process;
savings in transport energy and
better exploitation of the material (recycling).
The ISP process (23.4), together with processes according to the present invention which include the further processing of the material, leads to an innovative solution of the problem, characterized by the claims of the process and the production lines.
The present invention will now be described in greater detail with reference to an example of embodiment on the basis of the attached drawings in which: